In a field of electronics, for example, phenol resins, epoxy resins, glass epoxy resins, polyimide resins, polyester resins, polysulfone resins and polytetrafluoroethylene resins have heretofore been used as resin substrates from the viewpoints of mechanical strength, electrical properties, heat resistance and the like (for example, “ELECTRONICS JISSO GIJUTSU KOZA (Elementary Course of Mounting Techniques in Electronics), Vol. 1, Introduction”, edited by Association of Hybrid Microelectronics, Kogyo Chosakai Shuppan, issued on Jul. 1, 1998, Chapter 4, pp. 203-209).
In recent years, with the use of still higher frequency and speeding-up in the field of electronics, particularly, a lower dielectric constant has been required as material properties required of substrates. Attention has been attracted to porous resins as materials for resin substrates because they are low in dielectric constant compared with ordinary non-porous resin materials.
A material for connection between circuits or an anisotropically conductive material has a structure that perforations (hereinafter may also be referred to as “through-holes”) are provided in necessary portions of a substrate, and inner wall surfaces of the perforations are covered with a conductive material. In order to use a porous resin material as a material for these substrates, it is thus necessary to form perforations greater than the pore diameter of the porous resin material.
In general, methods for providing perforations (through-holes) in a substrate include machine-working methods, for example, punching by a punch and a die, blanking by a die, and perforating by a drill (for example, “MAIKURO KAKO GIJUTSU (Microworking Techniques)”, edited by the Editorial Committee of Microworking Techniques, The Nikkan Kogyo Shinbun, LTD., issued on Sep. 30, 1988, Chapter 1, pp. 8-13, Chapter 2, pp. 168-175). A method of perforating by ultrasonically vibrating the tip of a tool, a chemical etching method, in which a chemical corroding action is utilized to perforate, and a light-abrasion method, in which laser beams are irradiated to perforate, are also known as perforating methods.
When a porous resin material (hereinafter referred to as “porous resin base”) formed in the shape of a substrate is perforated by a machine-working method, however, the base itself is deformed, the porous structure of edges and inner wall surfaces of perforations is collapsed, and burr occurs on opening portions of the perforations, so that it is extremely difficult to form perforations with high precision while retaining the porous structure. Even when the method of perforating by ultrasonically vibrating the tip of a tool is applied to the porous resin base, it is difficult to form perforations with high precision.
When the porous resin base is perforated by irradiation of laser beams, peripheries of perforated portions are melted and deformed by heat, or the porous structure of edges and inner wall surfaces of perforations is collapsed. The chemical etching method permits a porous resin base to be perforated according to the kind of the resin forming the porous resin base. However, this method is unsuitable for a method for perforating a porous resin base composed of a corrosion-resistant resin. The porous resin base has a possibility that it may be perforated by irradiation of short-wavelength laser beams such as excimer laser. However, it takes a long time to work it, and so the cost thereof is expensive.
When the porous structure of edges and inner wall surfaces of the perforations in the porous resin base is collapsed, the properties characteristic of the porous resin material are impaired. The porous resin base has elasticity in a thickness-wise direction thereof. When the porous structure about the perforations is collapsed, however, the perforated portions are collapsed by only applying a compressive load to the porous resin base once to lose the elasticity.
When the porous resin base perforated is used as a material for connection between circuits or an anisotropically conductive material, it is necessary to make the inner wall surfaces of the perforations in a thickness-wise direction conductive by applying a conductive metal such as plating particles to them. When the porous structure about the perforated portions is collapsed, however, it is difficult to apply a plating catalyst. In addition, when the porous structure about the perforated portions is collapsed, the elasticity of the conductive portions is impaired even when the inner wall surfaces of the perforations are made conductive, so that the conductive portions themselves are collapsed when a compressive load is applied.
Further, even when the porous resin base is perforated, it is extremely difficult to selectively apply a conductive metal only to the inner wall surfaces of the perforations by a subsequent secondary working to make them conductive. As described above, it is difficult to precisely perforate the porous resin base, and the secondary working subsequent to the perforating is also difficult. These problems are specifically described taking an anisotropically conductive sheet (hereinafter may also be referred to as “anisotropically conductive film”) as an example.
In the field of electronics such as semiconductor devices, an anisotropically conductive sheet capable of imparting conductivity only to a thickness-wise direction thereof is used as a means for compactly conducting electrical connection between circuit devices. For example, the anisotropically conductive sheet is widely used for compactly conducting the electrical connection between circuit devices without using a means such as soldering.
There has also been proposed a method of interposing an anisotropically conductive sheet between electrodes to be inspected and electrodes of an inspection apparatus for the purpose of achieving electrical connection between the electrodes to be inspected formed on one surface of a circuit board, which is an object of inspection, and the electrodes of the inspection apparatus. This anisotropically conductive sheet preferably has elasticity in the thickness-wise direction thereof for the purpose of achieving the electrical connection between the electrodes to be inspected and the electrodes of the inspection apparatus without damaging the electrodes to be inspected and by absorbing variations of height among the electrodes to be inspected.
As specific examples of the anisotropically conductive sheet, there has been proposed, for example, an anisotropically conductive material for connection obtained by dispersing conductive particles in a binder composed of an epoxy resin to form a sheet (for example, Japanese Patent Application Laid-Open No. 4-242010). This anisotropically conductive material for connection is so constructed that when the conductive material is pressed between terminals opposed to each other, the conductive particles come into contact with the respective terminals only at compressed portions to conduct only in a thickness-wise direction between the terminals. The dispersed state of the conductive particles is controlled, thereby retaining the insulating property in a lateral direction of the sheet.
There have also been known anisotropically conductive sheets obtained by forming a great number of through-holes in a sheet formed from a polymeric material and filling a conductive material into the respective through-holes to make only specified portions of the sheet in a thickness-wise direction thereof conductive. There have been proposed, for example, anisotropically conductive sheets obtained by filling an insulating elastic polymeric substance, in which conductive particles have been dispersed, into each of a plurality of through-holes provided in an insulating plate formed from a resin material or a composite resin material reinforced with glass fiber and having stiffness to provide conductive path-forming devices (for example, Japanese Patent Application Laid-Open No. 9-320667).
There have been proposed electrically connecting members obtained by forming a great number of through-holes in an electrically insulating polymeric film and filling a metal into the respective through-holes to make the film conductive only in a thickness-wise direction of the film (for example, Japanese Patent Application Laid-Open No. 2-49385), and elastic connectors obtained by arranging a conductive member within a plurality of through-holes formed in a thickness-wise direction of an elastic sheet member subjected to a foaming treatment (for example, Japanese Patent Application Laid-Open No. 2003-22849).
In the anisotropically conductive sheets having the structure that the conductive material is filled into the respective through-holes in the sheet formed from the polymeric material, as a method for forming the through-holes (perforations), is adopted, for example, an etching method making use of a light source such as a laser, or a machine-working method such as pressing, punching or drilling. According to the etching method, small through-holes having a hole diameter of at most 100 μm, further at most 50 μm can be generally formed. However, this method is expensive in working cost. The machine-working method is generally used in the case where relatively large through-holes having a hole diameter of at least 100 μm are formed and has a feature that it is cheap in working cost.
On the other hand, the anisotropically conductive sheet desirably has sufficient elasticity to achieve connection between electrodes to be connected or electrodes to be inspected without damaging them and absorb variations of height among electrodes to be inspected to achieve good electrical connection. An anisotropically conductive sheet having elasticity in a thickness-wise direction thereof and permitting conduction in the thickness-wise direction under a low compressive load can be used repeatedly in inspection of electrical conduction because it has elastic recovery property in addition to the fact that it scarcely damages the electrodes to be inspected.
The anisotropically conductive sheets, in which an elastomer with the conductive particles dispersed therein or the metal filled into the respective through-holes in the sheet formed from the electrically insulating polymeric material to provide conductive portions (conductive paths), involve such problems that a high compressive load is required for achieving conduction in the thickness-wise direction and that the elasticity at the conductive portions is deteriorated due to deterioration of the elastomer with time or upon use under a high temperature in a burn-in test or the like.
In the state of the art, it has however been difficult even by those skilled in the art to use a porous resin base having elasticity in a thickness-wise direction thereof to form perforations with high precision without collapsing the porous structure and further to subject the porous resin base to a secondary working such as selective application of a conductive metal to inner wall surfaces of the perforations.
On the other hand, in a medical field, an expanded porous polytetrafluoroethylene (hereinafter abbreviated as “expanded PTFE”) is used in artificial blood vessels and medical devices such as patch repairing materials and sutures. The expanded PTFE has highly inert chemical properties and moreover has such properties that the internal growth of vital tissues is allowed by controlling a microstructure that the porous structure is formed. The expanded PTFE is known to facilitate the internal growth of vital tissues by providing microscopic perforations extending through in a thickness-wise direction thereof.
There have heretofore been proposed expanded PTFE sheet materials having a microstructure comprising nodes connected to each other by fibrils and having microscopic pores extending through in a thickness-wise direction thereof (for example, Japanese Patent Application Laid-Open (KOHYO) No. 8-506777 (through PCT route)). In this document, it is described that when an expanded PTFE material subjected to expanding before perforating is perforated by a needle, the perforations have very rough edges appearing to be caused by irregular cutting and deformation of the material. This document also shows that perforating by removing the expanded PTFE using a sharp blade also results in perforations having rough edges. When the expanded PTFE material subjected to the perforating is used as a medical device such as a patch, there is a possibility that some trouble may occur in a vital body when the perforations have rough edges.
Thus, the document (Japanese Patent Application Laid-Open (KOHYO) No. 8-506777 (through PCT route)) has proposed a method that the expanded PTFE material is not perforated, but an extruded product before expanding is perforated and then expanded. More specifically, this document discloses a process for producing an expanded PTFE material having a microstructure comprising nodes connected to each other by fibrils and microscopically perforated, which comprises extruding a billet preliminary formed from a mixture of PTFE and a liquid lubricant to produce an extruded product, removing the liquid lubricant from the extruded product, forming microscopic pores extending through in a thickness-wise direction of the extruded product and then uniaxially or biaxially expanding the extruded product. This document describes that when the extruded product before expanding is perforated and then expanded, an expanded PTFE material having perforations with substantially smooth edges is obtained.
According to the process described in the document (Japanese Patent Application Laid-Open (KOHYO) No. 8-506777 (through PCT route)), the extruded product before expanding is perforated and then expanded, thereby smoothening rough edges caused by the perforating. However, this process is insufficient to form perforations having edges highly smoothened. In addition, according to the process described in this document, the extruded product is perforated and then uniaxially or biaxially expanded to form a porous structure. It is thus difficult to control the positions and diameter of the perforations with high precision.
A perforated porous resin base used as a substrate of a material for connection between circuits or an anisotropically conductive material is required to preset the positions and diameter of a plurality of perforations with high precision. Unless the positions of the perforations can be controlled with high precision, electrical connection between circuit devices or electrical connection between electrodes to be inspected of a circuit board and electrodes of an inspection apparatus cannot be precisely carried out by means of such a porous resin base even when a conductive metal is applied to the inner wall surfaces of the perforations to make them conductive.
Further, according to the process described in the above-described document, an expanded PTFE material having perforations can be produced, but the process cannot be applied to selective application of the conductive metal to the inner wall surfaces of the perforations to make them conductive.